As a polyethylene filament with high strength, there is known a filament which is produced from an ultra-high molecular weight polyethylene by a so-called gel-spinning method and which has such a high strength and such a high elastic modulus that any of conventional filaments has never possessed, as disclosed in JP-B-60-47922, and this filament has already come into industrially wide use. This high strength polyethylene filament has advantages in its high strength and high elastic modulus. However, the high elastic modulus thereof sometimes induces disadvantages in various applications. For example, in case where the high strength polyethylene filament is used for ordinal cloth, the resultant cloth is very stiff to the touch and thus very unsuitable in view of fitness to one's body. In case where the high strength polyethylene filament is used for a bulletproof vest, it is demanded that the bulletproof vest should be made of a plurality of pieces of cloth superposed on one another so as to confront dangers which recently have been escalated more and more. As a result, the thickness of the cloth composing the vest is increased, so that one can not freely move in such a vest.
Under such circumstances, a filament which has a lower mass (METSUKE) and a very high strength is demanded.
In the meantime, a variety of olefin-based filaments and films recently have been used for separators for various batteries. In case where high strength polyethylene filaments are used as non-woven cloth or reinforcing materials for such separators, the high strength polyethylene filaments to be used are required to have such properties that can provide non-woven cloth with thin mass (METSUKE) and concurrently with a high strength maintained, in order to meet a demand for further compacting batteries.
JP-B-64-8732 discloses a filament which is made from an ultra-high molecular weight polyethylene as a starting material by so-called “gel spinning method” and which has a lower fineness, a higher strength and a higher elastic modulus than any of conventional filaments. However, the above production of the high strength polyethylene filament with a lower fineness by the gel spinning method uses a solvent, and the use of a solvent has a disadvantage of causing fusion of the filaments. Particularly in case where a very fine filament is desired, the drawing tension tends to increase with an increased spinning tension, which induces the fusion of filaments.
Japanese Patent No. 3034934 discloses a high strength polyethylene filament having a fineness of 16.7 dtex or less as a monofilament, which is produced by drawing a high molecular weight polyethylene having a weight-average molecular weight of 600,000 to 1,500,000. The fineness of the monofilament achieved in this patent is 2.4 dtex at least, and a high strength polyethylene filament having a fineness of 1.5 dtex or less which the present invention has achieved can not be obtained.
A high strength polyethylene filament produced by melt spinning is disclosed in, for example, U.S. Pat. No. 4,228,118. According to this patent, the high strength polyethylene filament disclosed has a strength of 17.1 cN/dtex, an elastic modulus of 754 cN/dtex, and a finness of 2.0 dtex at least as a monofilament of the fiber. Thus, a high strength polyethylene filament having a fineness of 1.5 dtex or less has not yet been obtained by the melt spinning.
One of commercially available polyethylene filaments made by the melt spinning has a tensile strength of about 10 cN/dtex at most, even though it is classified to high performance polyethylenes. At present, a polyethylene filament having a strength of as high as 15 cN/dtex or more has not yet been manufactured and put on the market.
The most effective solution to satisfy such a wide range of requirements is to decrease the fineness of a monofilament while maintaining the strength of the filament. However, the fineness of the monofilament of a polyethylene filament obtained by the melt spinning having a strength of as high as 15.0 cN/dtex or more is generally 2.0 to 5.0 dtex. Thus, it is impossible in a practical view to obtain as in the present invention not only a polyethylene filament which has a fineness of as low as 1.5 dtex or less, but also a polyethylene filament having a fineness of so far low as 1.0 dtex, at a productivity high enough for industrial production, even though such a filament can be present in a moment. Even if such a filament can be produced, the physical properties of the resultant filament markedly degrade and thus, this filament is insufficient for practical use. On the other hand, a high strength polyethylene filament having a fineness of as low as 0.5 dtex or less can be obtained by the gel spinning. However, such a high strength polyethylene filament with a lower fineness has problems in that there are many fusing points among each of the monofilaments thereof, and that it is very hard to obtain a desired uniform filament having a low fineness.
The present inventors assume that the following are the causes for the foregoing problems. In the melt spinning, the polymer has many intertwines of molecular chains therein, and therefore, the polymer extruded from a nozzle can not be sufficiently drawn. Further, it is practically impossible to use a polymer having a very high molecular weight of 1,000,000 or more in the melt spinning. Therefore, the resultant filament has a low strength even if achieving a low fineness. On the other hand, a high strength filament having a low fineness is made from a polyethylene having a molecular weight of as high as 1,000,000 or more, by the foregoing gel spinning, so as to decrease the number of the intertwines of molecular chains. This method has the following problems. The spinning and drawing tensions for obtaining a very fine filament becomes higher, and the use of a solvent for spinning and the drawing of a filament at a temperature higher than the melting point of the filament cause fusion in the filaments. Thus, a desired filament having an uniform fineness can not be obtained. Particularly in case where the cut fibers of such a filament is formed into non-woven cloth, the fused points of the filament degrades the physical properties of the resultant non-woven cloth. The present inventors have succeeded in obtaining a polyethylene filament having a very low fineness and a high strength which the gel spinning and the melt spinning could not achieve, and thus accomplished the present invention.
A high-strength polyethylene filament has advantages in a high strength and a high elastic modulus but has a disadvantage in low resistance to a compression stress because of its high crystallinity. In other words, the filament can well resist the tension in the filament axial direction, but it is destructed by a very low compression stress, if used in a situation under a compression stress.
As described above, a polyethylene filament with a high strength and a high elastic modulus made by the gel spinning is formed of crystals (having a high degree of order) from which defects are largely eliminated. Therefore, such a filament has very high physical properties but shows low resistance to a compression stress, as mentioned above. This fact is confirmed by an X-ray small angle scattering analysis in which no long period structure is observed.
Further, in case where an ultra-high molecular weight polyethylene having a molecular weight of 1,000,000 or more is used, it is possible to perform an ultra-drawing operation thereon. However, the structure of the resultant filament is so highly crystallized and ordered that no long period structure is observed in an X-ray small angle scattering pattern. Therefore, it is impossible to introduce a heterogeneous structure into the filament still maintaining the high physical properties.
The first object of the present invention is therefore to provide a high strength polyethylene filament which has a fineness of 1.5 dtex or less as a monofilament, a tensile strength of 15 cN/dtex or more, and a tensile elastic modulus of 300 cN/dtex, characterized in that the rate of dispersion-defective fibers cut from the filament is 2% or less.
Another object of the present invention is to provide a high strength polyethylene filament having a high resistance to compression which the conventional melt spinning and gel spinning are hard to impart the filament, a tensile strength of 15 cN/dtex or more, and a tensile elastic modulus of 300 cN/dtex or more, characterized in that a long period structure of 100 Å or less is observed in an X-ray small angle scattering pattern.